The present invention relates to a process for loading particulate filler complexes, and more particularly, increasing the deposition and retention of these filler complexes on fibers for manufacture of paper or paperboard products.
Inorganic material such as precipitated calcium carbonate (PCC) ground calcium carbonate (GCC), clay and talc are used extensively as fillers in the paper making process. Filler loading levels of 12-25% are typical in current paper making strategy to improve optical properties of the paper such as brightness and opacity. In some instances, the economics of substituting expensive fiber with inexpensive filler lends added incentive.
To insure that the fillers remain with the fiber web and ultimately with the paper product, retention aids are used. Normally retention aids are long chained polymeric compounds that flocculate the furnish and enhance filler-fiber xe2x80x9cattachment.xe2x80x9d However, high flocculation levels lead to non-uniformity in the web and poor paper formation.
To circumvent this, a method to attach the filler directly on to the fiber surfaces was described in French Patent 92-04474 and U.S. Pat. Nos. 5,731,080 and 5,824,364 to Cousin et al. In these patents a slip stream of pulp furnish is refined to low freeness ( less than 70 Canadian standard freeness [csf] vs. 450 csf, typically) and then treated to generate a highly loaded filler-fiber complex. When these complexes are recombined with untreated pulp, any desirable filler level can be targeted.
An alternative approach is described in U.S. Pat. No. 5,679,220 to Matthew et al. and U.S. Pat. No. 5,665,205 to Srivatsa et al. In both Srivatsa and Matthew the entire furnish is treated to nominal filler loadings without subjecting the pulp to high refining levels (low freeness). However, this procedure results in increases in capital and operating costs due to the treatment of larger pulp volumes. Accordingly, there is a need in the art to generate filler-fiber complexes easily and inexpensively as provided by the invention.
It is known in the art to produce fiber-filler complexes by contacting a fiber slurry with slaked lime and carbon dioxide gas to precipitate calcium carbonate (PCC). Such processes are described in the Cousin et al., Srivatsa, and Matthew et al. patents. The Cousin et al. patents describe a process for obtaining a fiber-based composite produced by precipitating calcium carbonate in situ in an aqueous suspension of fibers of expanded surface area having microfibrils on their surface. The crystals of precipitated calcium carbonate (PCC) are organized essentially in clusters of granules directly grafted on to the microfibrils without any binders or retention aids such that the crystals trap the microfibrils by reliable and non-labile bonding. Srivatsa et al. describes in situ precipitation on secondary fiber furnish. Whereas the Cousin et al. patents describe a batch reaction process, Matthew et al. describes a continuous process for forming fiber-filler complexes.
It is believed that the complexing process relies on anionic charges located on the fiber surfaces that act as nucleation sites to anchor the calcium carbonate crystal on to the fiber. The precipitating calcium carbonate physically binds on to the fiber at these sites. Typically, as you refine pulp, more surface area is generated and additional anchoring sites are created on the fiber.
The present invention provides for a source of fiber having a high surface area (anchoring sites) without the need for additional refining by obtaining them from process streams within the paper making process. The invention identifies sources of furnish that are suitable for fiber-filler complexes and provides methods to use them to improve the economics of the process and quality of product. It also targets improvements in paper machine operations, downstream processing of paper mill effluents and paper product characteristics.
Filler and fine recovery from the paper machine white water and other xe2x80x9cwastexe2x80x9d streams are known in the art. U.S. Pat. Nos. 5,558,782, 5,733,461, 5,830,364 and 6,004,467 to Bleakley et al. disclose treatments of xe2x80x9cwhite waterxe2x80x9d or waste streams in which an alkaline earth metal carbonate is precipitated to entrain particulate material present in the waste water. Methods to utilize the fines and filler material that escape the paper making process in paper mills are also described in the articles entitled xe2x80x9cA New Waste Conversion and Recycling Processxe2x80x9d by Olavi Toivonen (82nd Annual Meeting, Technical Section, CPPA pp.A101-107, 1996) and xe2x80x9cThe re-use of mineral and fines from paper mill waste streamsxe2x80x9d by RSE Martin and Dr. RD Cowling (1998 pp. 227-237).
PCT Publication WO 99/42657 published on Aug. 26, 1999 describes a process wherein xe2x80x9cwhite waterxe2x80x9d from a papermaking operation is separated into two componentsxe2x80x94xe2x80x9claden waterxe2x80x9d and xe2x80x9cclear waterxe2x80x9dxe2x80x94each of which component is incorporated, respectively, into separate compositions containing calcium bicarbonate and calcium hydroxide. These two compositions are combined to precipitate calcium carbonate on certain fibers.
U.S. Pat. No. 5,262,006 to Anderson discloses a process of making paper from pulp stock containing calcium sulfate wherein calcium carbonate is precipitated in situ by the addition of soluble calcium salt and carbon dioxide. U.S. Pat. No. 3,639,206 to Spruill discloses a method for treating waste water from an alkaline pulping operation by adding thereto calcium oxide or calcium hydroxide followed by carbon dioxide to form a precipitate which presumably entrains therein the fibrous waste materials in the suspension. The latter is removed from the suspension and ultimately disposed of.
EP 0604095 and EP 1052227 concern a processes for the treatment of aqueous suspensions of particular waste material which consists of precipitating an alkaline earth metal carbonate in the aqueous suspension such that the particulate material present in the aqueous suspension to be treated is carried into the alkaline earth metal carbonate precipitate.
EP 658606 discloses a process for the aggregation of waste solids in waste water from a paper manufacturing factory in which the waste water recovery system includes at least one step in which an alkaline earth metal carbonate is precipitated in the aqueous suspension constituting the waste water to form a mixed aggregate material.
WO 99/03928 published on Jan. 20, 1999 describes a process for precipitating crystals of an insoluble white pigment compound in an aqueous medium containing dispersed particles of fine particulate material and fibers to form a composite pigment material comprising a composite fiber matrix, precipitated pigment crystals and fine particulates dispersed and bonded in the matrix.
WO 00/39029 published on Jul. 6, 2000 discloses a process for increasing the solids concentration of a dilute aqueous suspension of a particulate carbonate, especially calcium carbonate, to form a concentrated suspension which is fluid enough to enable it to be pumped and delivered through pipes or hoses but viscous enough to prevent the formation of a sediment of the coarser particles present.
From the prior art mentioned it is seen that processes for filler and fine recovery are known, however, the prior art does not provide a method for combining separated process streams, including fines and/or filler, with long fibers. It would be appreciated that advantage over known applications would be obtained by providing a method for manufacture of loaded paper or paperboard products comprising separating at least one process stream from the papermaking process; wherein said process stream includes fines; combining the process stream with long fibers and thickening to form a residue; and treating the residue to form fiber-filler complexes. These fiber-filler complexes are then used in the papermaking process to form the paper.
Accordingly, it is a broad object of the invention to provide a method to produce fiber-filler complexes and enhance the deposition and retention of these complexes on fibers for the manufacture of paper or paperboard products.
An object of the invention is to provide a source of fiber having high surface area without the need for refining.
A more specific object of the invention is to separate process streams from various points within the paper making process to obtain primary and/or secondary fines for subsequent treatment to form fiber-filler complexes used in the papermaking process to form paper.
A specific object of the invention is to provide paper which has enhanced stiffness properties, enhance filler retention and uniform z- and cross direction filler profiles.
A more specific object of the invention is to provide target filler levels in the paper that are at least 5% and preferably higher than 20%.
In general, the invention provides a method for manufacture of loaded paper or paperboard products, comprising the steps of separating at least one process stream from the papermaking process, wherein the process stream includes fines. Combining the process stream with long fibers and thickening to form a residue. Treating the residue to form fiber-filler complexes and using the fiber-filler complexes in the papermaking process to form the paper.
The process streams used in the invention process can be separated from any point along the papermaking process, including points prior to the paper machine, from the paper machine or after the paper machine. The process streams separated may include primary or secondary fines and depending on the stream separated may further include filler particles.
The process stream is combined with long fibers, typically having a length greater than 0.1 mm. The long fibers are from any source and can be natural or synthetic. In alternate embodiments, the long fibers are refined to produce greater fiber surface area.
After the combination of the long fibers with the process stream, the material is thickened to form a wet residue. This residue is preferably treated with calcium and carbonate ions to effect crystallization of calcium carbonate in situ. The crystals of calcium carbonate bond to the fines, long fibers and/or fillers which may be present in the residue. The crystals of calcium carbonate, CaCO3 (PCC), are organized essentially in clusters of granules directly grafted onto the fines, filler and/or fiber present in the residue, without binders or retention aids present at the interface between said crystals of PCC and the fines, filler and/or fiber, so that the majority of said crystals of PCC trap said fines, filler and/or fiber by reliable and non-labile bonding to form the fiber-filler complex.
In an alternate embodiment, the residue formed is treated with particulate filler material to form the fiber-filler complexes. The particulate filler material used may be selected from the group consisting of inorganic pigments, organic pigments, organic latices and hollow spheres. Particularly, the pigments are selected from the group consisting of talc, clay, TiO2, calcium carbonate, silica based pigments and aluminum based pigments.
The resulting paper formed has enhanced stiffness properties, enhanced filler retention and has uniform z- and cross direction filler profiles.
Other objects, features and advantages of the present invention will be apparent when the detailed description of the preferred embodiments of the invention are considered with reference to the drawings, which should be construed in an illustrative and not limiting sense as follows: